This invention relates to a calibration system for a printing plate picture pattern area meter which measures the area of a printing image (or a picture pattern) from a printing plate for an offset press.
In order to measure a picture pattern area, a method of measuring a picture pattern area from a printing plate and methods in which a proof, a print, a reflecting original and a transparent original area used for detection, are available. The measurement value of a picture pattern area is utilized to check the density of a print during the operation of a printing machine, thereby to carry out feedback control, or the measurement value is utilized to preset the quantity of ink before the printing operation is started.
The picture pattern of an ordinary printing plate is such that a plurality of originals are printed thereon. Therefore, in the case where those other than a printing plate and a print are used for measurement of a picture pattern area, after each picture pattern area has been measured the measurement data must be added up with the layout on the printing plate taken into consideration. On the other hand, in the case where a printing plate and a print are used for the measurement, the measurement data can be utilized immediately. However, in the case where the measurement data are obtained from the print, a feedback control system for correcting variations due to external disturbances caused during the printing operation must be employed, because the measurement data have been obtained after the printing operation started. On the other hand, in the case where the measurement data are obtained from the printing plate, the degrees of opening of ink adjustment keys are preset before the printing operation starts, to provide satisfactory prints from the beginning of the printing operation.
Examples of the device in which a printing plate is utilized for measurement are as follows: In a first example of the device, a printing plate wound around a cylinder is rotated at high speed, and the average residual image of divisional areas thereof are measured (cf. Japanese Patent Application Publication No. 42205/1972). In a second example of the device, the original plate (or the printing plate) of an offset press is scanned to obtain the number of pulses which corresponds to the area of the printing image region, thereby to control the quantity of ink to be supplied (cf. Japanese Patent Application Laid-Open No. 53804/1973). In a third example of the device, the printing image of a printing plate is detected and integrated for every ink adjustment key, the quantity of light reflected from the non-printing-image is detected by using an auxiliary printing plate, and a signal for the printing image region only is calculated from the detection signals of the printing plate and is converted into a value corresponding to the printing image area, to control the quantity of ink (cf. Japanese Patent Application Laid-Open No. 67714/1974). In a fourth example of the device, an original plate is scanned longitudinally to measure the percentage of occupation of the printing image in the widthwise direction, so that the quantity of ink to be supplied is controlled in ink roller section (cf. Japanese Patent Application Laid-Open No. 2505/1976). In a fifth example of the device, a printing plate is longitudinally scanned with a photo-electric detector device which is set at a predetermined position in the lateral direction of the printing plate surface, to obtain the total quantity of ink, thereby to control the quantity of ink (cf. Japanese Patent Application Publication No. 47405/1972).
However, none of these conventional devices can perform the detection with high accuracy, and accordingly they are not practical in use. This will be described in more detail. After a transparent film original is printed on a printing plate through contact exposure, in general the printing plate is processed according to processing steps shown in FIG. 1. A positive type printing plate usually employed is developed (Step S1) in order to remove the photo-sensitive layer of a region (non-printing-image region) which has been exposed to light, thereby to make an ink repellent surface. After the developing treatment, the remaining unnecessary photo-sensitive layer is dissolved with an erasing solution (Step S3). After being dried (Step S4), an affinitizing solution is applied to the entire printing plate (Step S5). Then, the printing plate surface is dried through buffing to the extent that no affinitizing solution is left on the printing plate surface (Step S7). The affinitizing and buffing treatments are preparatory treatments which are carried before a burning treatment (or high temperature heat treatment) is carried out. By performing the burning treatment (Step S8), the printing withstanding characteristic of the print plate is increased twice or thrice. In order to protect the surface of the non-printing-image region and to increase its hydropholic characteristic, a so-called "gumming treatment" is carried out (Step S10). Thus, the printing plate has been completely processed. In these processing steps, the burning treatment is considerably effective in improving the printing withstanding characteristic of the printing plate. However, in the burning treatment, the printing plate is heated at a high temperature 250.degree. C. to 300.degree. C., and therefore the ordinary printing plate whose base is aluminum is deformed by heat, and it remains deformed even after it has cooled; that is, the flatness of the printing plate surface is degraded. Accordingly, when the quantity of reflected light is detected in order to obtain a picture pattern area from such a printing plate, the quantity of reflected light varies depending on the picture pattern area and according to the degree of roughness of the printing plate and to the amount of displacement of the printing plate. Thus, the detection cannot be made with high accuracy, and accordingly it is impossible to accurately determine the quantity of ink to be supplied.
In a printing factory having a number of printing machines of different kinds, in general a variety of printing plates are handled in the printing operation. Therefore, it is more preferable to use the picture pattern area meter on the printing line in the printing site so that one measuring device measures data for performing the ink adjustments of a number of printing machines than to use it with a single printing machine in the printing site.